Add note about PoW constant validation. (#4476)

* Update mixhash

Signed-off-by: Tim Beiko <t.beiko23@gmail.com>

* Revert change, add better note

Signed-off-by: Tim Beiko <t.beiko23@gmail.com>

EIPS/eip-3675.md

@@ -85,6 +85,8 @@ Beginning with `TRANSITION_BLOCK`, the validation of the block's **`extraData`**
 
 *Note*: Logic and validity conditions of block fields that are *not* specified here **MUST** remain unchanged. Additionally, the overall block format **MUST** remain unchanged.
 
+*Note*: Subsequent EIPs may override the constant values specified above to provide additional functionality. For an example, see [EIP-4399](./eip-4399.md).
+
 
 ### Block validity
 
@@ -98,7 +100,6 @@ Beginning with `TRANSITION_BLOCK`, the block validity conditions **MUST** be alt
 
 *Note*: Validity rules that are not specified in the list above **MUST** remain unchanged.
 
-
 ### Block and ommer rewards
 
 Beginning with `TRANSITION_BLOCK`, block and ommer rewards are deprecated. Specifically, the following actions **MUST** be taken: